Definition.—Cubosphærida with one simple lattice-sphere and six simple spines in pairs different; the two opposite spines of each pair equal, the three pairs unequal.
The genus Hexastylidium differs from its probable ancestral form, Hexastylus, by the unequal growth of the six simple spines; the two spines of each pair reaching the same dimensions, whilst the three pairs are different. They correspond therefore to the three axes of a rhombic crystal.
1. Hexastylidium rhomboides, n. sp.
Shell thin walled, smooth, with irregular, polygonal pores and very thin bars. Its form is not, as commonly, a sphere, but a rhombic octahedron. The radial proportion of the three unequal dimensive axes = 1 : 2 : 4. The radial proportion of the three pairs of spines = 1 : 3 : 8. Spines thin cylindrical, at the base angular.
Dimensions.—Diameter of the major shell axis 0.2, middle 0.1, minor 0.05; length of the major spines 0.3, middle 0.12, minor 0.04.
Habitat.—Indian Ocean, Madagascar, Rabbe, surface.
2. Hexastylidium spirale, n. sp.
Shell thick walled, spherical, thorny, with irregular, roundish pores, three to five times as broad as the bars; six spines very stout, prismatic (as broad as one large pore), with three ring-like, spirally convoluted edges. Two opposite major spines of extraordinary length, ten to twelve times as long as the shell diameter, the two middle opposite spines being about as long as the latter, the two minor scarcely one-third as long. (Similar to Hexastylus spiralis, Pl. [21], fig. 7, but distinguished by the very unequal length of the spines.)
Dimensions.—Diameter of the shell 0.12; length of the major spines 1 to 1.5 mm., middle 0.15, minor 0.04.
Habitat.—Central Pacific, Station 266, surface.